This invention pertains to cardiac pacemakers and methods for operating such devices. In particular, the invention relates to methods for employing pacing therapy to maintain hemodynamic stability.
Ventricular tachyarrhythmias, in which the ventricles beat more rapidly and irregularly than normal, can be due to a variety of etiologies. One common cause, for example, is atrial fibrillation where the atria depolarize in a chaotic fashion with no effective pumping action. The intrinsic ventricular rhythm that occurs during an episode of atrial fibrillation is a result of the chaotically occurring depolarizations occurring in the atria being passed through the AV node to the ventricles. The intrinsic ventricular rate is thus governed by the cycle length of the atrial fibrillation and the refractory period of the AV node. Although the intrinsic ventricular rate is less than the atrial rate, due to the refractory period of the AV node, it is still rapid and irregular. When the ventricles contract at irregular intervals, the contraction can occur prematurely before diastolic filling is complete which decreases the stroke volume for that contraction. This can be especially significant in, for example, congestive heart failure patients who are already hemodynamically compromised. Concomitant atrial fibrillation where the atria no longer act as effective priming pumps can also contribute to the problem. An irregular ventricular rate can thus depress cardiac output and cause such symptoms as dyspnea, fatigue, vertigo, and angina. An objective of the present invention is to use pacing therapy to maintain hemodynamic stability in the presence of an irregular intrinsic ventricular rhythm.
The present invention is a system and method for regularizing the ventricular rate by adjusting the lower rate limit of a pacemaker in accordance with changes in the measured intrinsic ventricular rate. By making the ventricular escape interval track a mean interval between intrinsic beats, less variability in the overall ventricular rhythm is allowed by the pacemaker. Because the ventricular rate regularization keeps the ventricular escape interval close to the interval between intrinsic beats, however, the risk of delivering a pace coincident with an intrinsic depolarization is increased. Such a pace produces a fusion beat which is hemodynamically inefficient and tends to counteract the beneficial effects of ventricular rate regularization in maintaining hemodynamic stability. In accordance with the invention, the risk of producing fusion beats is reduced by providing a ventricular sensing channel for each ventricle and inhibiting ventricular pacing when a ventricular depolarization is detected from either ventricle before the expiration of the ventricular escape interval. Such biventricular sensing is employed whether the pacemaker is operating in a univentricular or biventricular pacing mode.